Replication of DNA summary

Replication of DNA


Section content

Mechanism of DNA replication “3 hypotheses”
Semi-conservative hypothesis “Evidence…”
Semi-conservative DNA replication “Mechanism”
Enzymes “What role do enzymes play?”


1. Conservative
hypothesis


In this hypothesis…

Both strands of DNA act as templates.
The parent DNA is intact & goes into one daughter cell.
The new DNA molecule goes into the other daughter cell


. Dispersive
hypothesis


In this hypothesis…

Parental DNA molecule breaks into short segments, which acts as templates for synthesis.
The segments are then join together.
Results in two DNA molecules with old & new segments in each strand.


3. Semi-conservative
hypothesis


In this hypothesis…

Both strands of parental DNA separate & act as templates.
New bases join each old strand to become a new strand.
Results in two DNA molecules that are hybrids of old & new strands.


Evidence by
Meselson & Stahl

The result
For the 1st generation cells, all DNA bands were between those of 14N & 15N. They were a hybrid of 14N & 15N strands


For 2nd generation cells, half of the DNA molecules were of the hybrid type. The other half were pure 14N DNA.

The results confirm that DNA replication is semi-conservative

2. Replication of DNA


Free deoxyribonucleotides


synthesised in the cytoplasm
transported into the nucleoplasm via pores in the nuclear envelope


Helicase (an Enzyme)


Causes DNA molecule to unwind
Begins at sites called origins of replication
Hydrogen bonds between bases break


Single-stranded DNA binding protein


stabilises the unwound single strands of DNA.

Therefore allows the unwound region to serve as a template.

Leading Strand
Synthesized continuosly in 5’ to 3’ direction

Primase (an enzyme)
Catalyses the synthesis of a short RNA chain.
RNA chain is called primer.
Primer is complementary to DNA template.


DNA polymerase III (an enzyme)
Catalyses the bond formation between free deoxyribonucleotides & template.
Ensures complementary base pairing.



Lagging Strand
Synthesized in short fragments
known as Okazaki fragments


DNA ligase (an enzyme)
Catalyses the phosphodiester bonds between two adjacent Okazaki fragments.
Requires a free hydroxyl group at 3’ end of one DNA chain.
And a phosphate group at the 5’ end of the other DNA chain.
New DNA
New DNA results when the new parental-daughter pairs of DNA rewind’